Printed circuit boards typically include a mat of woven glass fibers within a cured resin substrate. The glass fibers provide structural reinforcement for the resin. The glass fibers are formed by extruding molten glass. Occasionally, a bubble in the molten glass is carried into the extrusion process. In such cases, the bubble can be contained in a formed thread in an elongated form. As a result, the thread includes a hollow region.
If a hollow thread is used in the manufacture of a printed circuit board, the hollow thread could cause a circuit failure, such as a short circuit or an open circuit. For example, holes or vias are often drilled through a printed circuit board. If such a hole is drilled through a hollow thread, a conductive circuit material could travel through the hollow portion of the thread, forming a conductive anodic filament (CAF) that may cause an electrical failure in the printed circuit board. As the density of circuit elements on printed circuit boards increases, the likelihood that a hollow thread will cause a circuit failure also increases. Thus, reducing or eliminating the presence of hollow threads in printed circuit boards is important to reduce the number of faulty circuit boards.
Hollow fiber detection is difficult once the glass fibers are formed into glass cloth. The glass cloth is typically stored in long rolls (e.g., 2 kilometer rolls), of which only a small portion (e.g., the first 10 centimeters) can be feasibly tested for the presence of hollow fibers. Then, the hollow fiber count for the small sampled portion may be extrapolated onto the entire roll of glass cloth. Such sampling methods do not offer a reliable nor reasonable measure of an amount of hollow fibers that are present in a particular sheet of pre-impregnated (prepreg) material that is used to form a printed circuit board.